1. Field of the Invention
The present invention relates to a driving method for a liquid crystal device, e.g. a liquid crystal display, in which a liquid crystal is disposed between a first substrate provided with a unidirectionally aligned data-electrode group and a second substrate provided with a select electrode group aligned perpendicularly to said data-electrodes.
2. Description of the Related Art
Liquid crystal displays (LCDs), using liquid crystals as displays, have several -advantages, e.g. low electric consumption and thin and lightweight appearances, and have been widely used in clocks, watches, electronic calculators, computer displays and television (TV) sets.
Use of ferroelectric liquid crystals (FLC) in LCDs has been intensively studied. Meyer first synthesized a FLC in 1975, and Clark and Lagerwall invented a surface stabilized FLC capable of domain inversion by an electric field in 1980. A FLC molecules has a permanent dipole moment perpendicular to the longitudinal axis of the molecule, produces spontaneous polarization, and is capable of switching by means of an electric field. FLC displays have the following advantages:
(1) They respond 1000 times faster than twisted nematic (TN) liquid crystal displays due to .mu.-second switching rates; PA1 (2) They have large angles of visible field because the molecules do not basically form twisted structures; and PA1 (3) They can maintain an image when they are de-energized, have memory effects of the image, and are capable of passive-matrix driving of 1,000 or more scanning lines in high definition TVs. PA1 wherein after a select pulse is applied to the select electrodes, a pause corresponding to at least one line is provided before a data-pulse sequence is applied to the data-electrode group. PA1 wherein a select pulse applied to the select-electrode group is synchronized with a data-pulse sequence applied to the data-electrode group by shifting them a half line (H/2) relative to each other so that the select pulse and the data pulse have opposite polarity in relation to each other. PA1 wherein a select pulse is applied to the select-electrode group while a data pulse, in which the time and/or the voltage is determined so as to offset the effects of the reversed electric field generated during switching of the liquid crystal, is applied to the data-electrode group. PA1 wherein a select pulse applied to the select-electrode group is synchronized with a data-pulse sequence applied to the data-electrode group by shifting them a half line (H/2) relative to each other so that the select pulse and the data pulse have opposite polarity in relation to each other, and a select pulse is applied to the select-electrode group while a data pulse, in which the time and/or the voltage is determined so as to offset the effects of the reversed electric field generated during switching of the liquid crystal, is applied to the data-electrode group.
Therefore, FLC displays are suitable for trends required for displays toward high definition, low cost, and large screens.
A typical FLC display has a configuration as schematically shown in FIGS. 1 and 2. Transparent electrode groups 2a and 2b, which are composed of indium tin oxide (ITO) with a surface resistivity of 100 .OMEGA., are provided on transparent glass substrates (Corning 7059, 0.7 mm thick) 1a and 1b, respectively. The transparent electrode group 2a is provided as a data electrode (column electrode) group and the transparent electrode group 2b is provided as a scanning electrode (row electrode) group. Each electrode group is patterned into a stripe by etching, and the two electrode groups 2a and 2b are perpendicularly arranged in relation to each other.
Rhombic SiO deposited films 3a and 3b are formed as liquid crystal alignment films on the transparent electrode groups 2a and 2b, respectively. In order to form SiO rhombic deposited films, each substrate is disposed just above a SiO deposition source in a vacuum evaporation system, so that the angle between the normal line of the substrate and the vertical line from the deposition source is 85 degrees. After SiO is deposited on the substrate at a temperature of 170.degree. C., the substrate is baked at 300.degree. C. for 1 hour.
A pair of substrates 1a and 1b having alignment films are assembled so that the data-electrode group 2a and the scanning electrode group 2b are perpendicular to each other and directions of alignment treatment of the two electrode groups are antiparallel. Glass beads 4 (diameter: 0.8 to 3.0 .mu.m, made by Catalysts & Chemical Industries Co., Ltd.) are used as spacers to secure a given gap distance. Although the directions of alignment treatment are antiparallel in this case, two substrates can also be assembled so that the directions are parallel.
In substrates having small areas, the gap between the two substrates is controlled to a given distance such that spacers 4 having a desirable diameter are dispersed into a sealing agent 6 (a UV curable bonding agent, trade name: Photorec made by Sekisui Chemical Co., Ltd.), which is used for bonding the peripheries of the substrates, in an amount of approximately 0.3 percent by weight. In substrates having large areas, glass beads are distributed on the substrates in an amount of 100 /mm.sup.2 to secure a gap, and the periphery of the cell other than an injection port for the liquid crystal is bonded with a sealing agent 6.
A liquid crystal composition, in which, for example, a ferroelectric liquid crystal 5 (YS-C152 made by Chisso Corporation) is homogenized with an ultrasonic homogenizer at an isotropic phase temperature and injected between the two substrates 1a and 1b. The ferroelectric liquid crystal composition is injected in a flowable state, for example, at an isotropic phase temperature or a chiral nematic phase temperature under a reduced pressure. The injected liquid crystal is gradually cooled, the overflowed liquid crystal around the injection port is removed, and the injection port is sealed with an epoxy bonding agent to prepare a FLC display 11.
The FLC display 11 is generally driven by an X-Y passive-matrix system. In the NTSC system, 1H (a scanning time per horizontal scan or a selection time per line) is 63.5 .mu.sec., and each selection pulse is 63.5/2 .mu.sec. due to bipolar voltage application in view of the electrical neutralization condition. As shown in FIG. 3, select pulses are applied as a threshold voltage through the row electrode group 2b and data-pulse sequences are applied through the column electrode groups 2a.
In ferroelectric liquid crystal devices, e.g. surface stabilized ferroelectric liquid crystal devices, the alignment direction of a molecule M switches between two states, i.e., state 1 and state 2 shown in FIG. 4, in response to the external electric field E (in FIG. 4, the symbol Ps represents spontaneous polarization). Such a change in molecular alignment results in a change in transmittance when the liquid crystal device is provided between two polarizers perpendicular to each other, as shown in FIG. 5, in which the transmittance steeply changes from 0% to 100% at a threshold voltage V.sub.th of the applied voltage. The voltage width in the transmittance transition region is generally 1 V or less.
As de scribed above, in a ferroelectric liquid crystal display using a conventional bistable mode, only these two states are stable, and thus it is difficult to hold a stable intermediate transmittance. Gradation display by means of voltage control therefore is achieved with difficulty or cannot be achieved.
Some gradation methods are proposed, i.e., an a real gradation method in which the gradation of an image is adjusted with subpixels, and a time integration gradation method in which gradation is performed by repeated switching operations in one field. These methods, however, do not achieve satisfactory gradation display and result in high production cost because the gradation display is not performed in one pixel.
Analog gradation methods proposed for performing gradation in one pixel include methods for imparting a localized gradient to the field intensity by means of a change in the distance between the opposing electrodes in one pixel or a change in the thickness of the dielectric layer formed between the opposing electrodes; and methods for imparting a gradient to the voltage by means of the change in the electrode material. Production of liquid crystal display devices having practical analog gradation characteristics by means of the above-mentioned methods, however, requires complicated processes and severely controlled production conditions, and results in high production cost.
The ferroelectric liquid crystal display is driven by a passive X-Y matrix system at a run time of, for example, 63.5 .mu.sec. for the NTSC system and a selection pulse width of 63.5/2 .mu.sec. as shown in FIG. 3, because a bipolar voltage is applied instead of a DC voltage. Selection pulses as a threshold voltage are applied through the row electrodes and data-pulse sequences are applied through the column electrodes. The voltage of the data pulses is varied in order to generate a grey scale by the analog gradation as shown in FIG. 6. The data pulses are thereby always applied as bias pulses to th e en tire frame.
The present inventors discovered that the threshold voltage of a pixel shifts with gradations of its adjacent pixels. Such voltage shift is generally noticeable and not negligible in analog gradation display as shown in FIG. 7.
In particular, a voltage corresponding to a reversed electric field remaining in the current display pixel is added to a data pulse which is applied to the next line. A considerably high electric field having opposite polarity is thereby generated and will cause switching in domains having low threshold voltages. That is, the applied voltage including the voltage due to reversed electric field is higher than the threshold voltage.
For example, in a data-pulse sequence DP' in FIG. 10 described in detail below, immediately after switching by a select pulse, a current displaying pixel PX.sub.1-1 on a line 2b.sub.1 for displaying a grey level is affected by the reversed electric field when a data pulse is applied to the next line 2b.sub.2 for displaying a white level, and the grey level of the current displaying pixel PX.sub.1-1 will change to a black level by means of the data pulse for the next line.
The present inventors recognize that the effect of the following lines and in particular the next line on the gradation of the current displaying pixel is a factor greatly inhibiting analog gradation display.
Liquid crystal compositions having high bias voltage stability have been developed in view of improving the anisotropic dielectric constant, in order to reduce the effect of the data voltage in the passive matrix. It is, however, important to prevent the reversed electric field generated by spontaneous polarization of the composition in order to reduce the effect of the next line.
Liquid crystal display devices must satisfy many characteristics other than the gradation, such as a operational temperature range, a preservation temperature range, contrast, a response speed, a hysteresis width and a threshold voltage width generating a gradation. Control of the effects due to the bias voltage by the composition is therefore limited. Herein, the term "hysteresis" means lagging of transmittance behind applied field intensity.
The present inventors have studied the above-mentioned phenomenon and discovered that one of effects of the bias pulse is a reversed electric field which is generated by the memory effect of the spontaneous polarization of the ferroelectric liquid crystal. The phenomenon will now be described in detail.
FIG. 8 is a graph illustrating the change in transmittance of one pixel in a liquid crystal display device when the color of the adjacent pixel in the next line is changed. As shown in FIG. 8, the transmittance noticeably varies with the data voltage of the next line. The results demonstrate that the transmittance cannot be uniquely determined by the applied voltage and thus the number of gradations decreases.
It has been generally considered that anisotropy in the dielectric constant of liquid crystal molecules having memory effects causes adverse effects of the bias voltage. The present inventors, however, have discovered that transmittance is affected primarily by spontaneous polarization rather than the anisotropic dielectric constant.